Control valves are typically operated by pneumatic actuators, such as spring and diaphragm actuators, that are directly coupled to the valve. The pneumatic actuator provides the force necessary to move a valve plug to control a fluid flowing through the valve. As understood by those skilled in the art, increasing or decreasing the air pressure within a pressure chamber defined by the diaphragm and the actuator casing, creates an air spring that moves the diaphragm. An actuator rod is attached to the center of the diaphragm and is guided along its longitudinal axis through the actuator housing by a journal bearing. By attaching the actuator rod to the diaphragm, direct changes in actuator air pressure create corresponding changes in the axial position of the actuator rod.
The actuator rod is attached to a valve stem that protrudes from the valve body through the valve bonnet. By mechanically coupling the actuator rod to the valve stem, the position of the attached valve plug can control the fluid flowing through the valve. Typically, the valve stem connector consists of a single, rigid connector that includes two threaded cavities for receiving the actuator rod and the valve stem. Conventional valve stem connectors suffer certain manufacturing disadvantages and design limitations as described below.
FIG. 1 shows a cross-sectional view of typical valve stem connector 10 mechanically fastening the actuator rod 30 to the valve stem 40 of the valve assembly 100. The actuator 48 (partially depicted by the actuator yoke legs) is attached to the valve body 50 by threadably engaging the yoke lock nut 70 to the valve bonnet 64. When the supplied air pressure to the actuator 48 changes, the actuator rod 30 moves correspondingly along a longitudinal axis 90. The valve stem connector 10 couples the axial motion supplied by the actuator rod 30, through the valve stem 40, to the valve plug 46 allowing the valve plug to be positioned relative to the valve seat 60. For example, when the valve plug 46 is positioned away from the valve seat 60, fluids can flow from the valve inlet 52 to the valve outlet 54 as indicated by the arrows shown.
The valve assembly 100 uses valve packing 58 to seal fluids inside the valve body 50 and also provides a guiding surface that is substantially parallel to the valve stem 40. The valve packing 58 is compressively loaded within the valve packing box 61 in the valve bonnet 64 by a packing flange 72, packing studs 69a-b, and packing nuts 66a-b. The compressive load exerted upon the valve packing 58 circumferencially expands the valve packing 58 to create the fluid seal and the guiding surface for the valve stem 40. Accordingly, any asymmetric forces exhibited by the valve stem 40 upon the valve packing 58 can increase packing friction through the region of asymmetric force causing degradation and erosion of the valve packing 58 and substantially reduce its useful life. Conventional valve stem connectors 10 cannot eliminate these asymmetric forces. The presence of these asymmetric forces can increase maintenance costs and can increase the risk of environmental hazards if the fluids escape to the surrounding atmosphere.
Referring to FIGS. 1 and 2, the conventional valve stem connector 10 is comprised of two connector halves 12a-b that are fastened by bolts 14a-b inserted through clearance holes 13a-b and into bolt holes 15a-b to form a rigid connector. The valve stem connector 10 has a threaded upper cavity 20 to “contain” the actuator rod 30 as will be described below. The threaded lower cavity 22 of the valve stem connector 10 “contains” the valve stem 40. The actuator rod 30 and the valve stem 40 are joined when the upper threads 62 and lower threads 68 mechanically engage the corresponding external threads 32 and 38 on the actuator rod 30 and the valve stem 40, respectively. The tapered internal surface 18, resultant of the manufacturing operation and tapered at approximately 45 degrees, connects upper cavity 20 to lower cavity 22. The actuator rod 30 is typically larger in diameter than the valve stem 40 as shown in FIG. 1. Due to conventional design techniques, the valve stem connector 10 can only accommodate small variations in length of the actuator rod 30 and valve stem 40. As appreciated by those familiar with these types of connectors, valve and actuator manufacturers generally resolve this limitation by designing and maintaining a broad product line with numerous combinations of valves and actuators to meet a particular application.
To complete the valve assembly 100, the actuator rod 30 and the valve stem 40 are threaded in counter-rotating directions into cavities 20 and 22 until the desired length is achieved. The travel length of the valve assembly 100 must be set prior to operation and is typically achieved through adjustment in the valve stem connector 10. The adjustability of the valve stem connector 10 is limited in one direction when the end surface 44 of the valve stem 40 makes contact with the end surface 24 of the actuator rod 30. In the opposite direction, the adjustment limitation is based on the a minimum number of threads that must be engaged to create a secure and safe mechanical connection. Conventional valve stem connectors 10 offer an adjustment range of approximately 0.25 to 0.50 inches. Finally, bolts 14a-b are further tightened to create additional compressive load between the threads 32, 38, 62, and 68 to secureably fasten actuator rod 30 to valve stem rod 40.
Since the valve stem 40 and the actuator rod 30 are coupled by threading both into a single rigid connector, narrow manufacturing tolerances on the valve stem connector 10 must be maintained. Any misalignment due to under-toleranced connectors halves 12a-b may further increase the asymmetric forces and therefore the friction experienced by the valve packing 58. The narrow manufacturing tolerances result in higher manufacturing costs and increased assembly difficulty. More significantly, when using a conventional valve stem connector 10, the connection between the actuator rod 30 and the valve stem 40 is rigid. The rigidity of the connection requires the actuator rod 30 and the valve stem 40 to be perfectly aligned along a common longitudinal axis 90 to eliminate valve packing 58 wear. As understood by those skilled in the art, industry standard manufacturing tolerances and typical assembly methods for the actuator 48 and the valve body 50 cannot provide precise alignment of the actuator rod 30 and the valve stem 40.